Metabotropic glutamate receptors are classified into 3 groups according to the sequence homology, signal transduction mechanisms and pharmacological properties. Among them, group 2 metabotropic glutamate receptors (mGlu2 and mGlu3 receptors) are G protein-coupled receptors that bind to adenyl cyclase and suppress the phosphocholine-stimulated accumulation of cyclic adenosine monophosphate (cAMP) (Non Patent Literature 1). Also, the group 2 metabotropic glutamate receptors exist mainly in the presynapses of the glutamatergic nervous system and function as autoreceptors, thus suppressing excessive release of glutamic acid (Non Patent Literatures 2 and 3). It is considered that compounds antagonizing group 2 metabotropic glutamate receptors may be effective for treatment or prevention of acute and chronic neuropsychiatric diseases and neurological diseases. (1R,2R,3R,5R,6R)-2-Amino-6-fluoro-3-alkoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives and (1S,2R,3R,5R,6S)-2-amino-3-alkoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives are compounds having a strong antagonistic effect on group 2 metabotropic glutamate receptors. For example, MGS0039 is disclosed as such a compound. Its antagonistic activity is 20 nM (mGlu2 receptor) and 24 nM (mGlu3 receptor), and it has been reported that 1 mg/kg of the compound is sufficient to suppress immobility time, as with existing antidepressants, in the forced swimming test of rats as depression animal models. It has been further reported that the compound also shortens immobility time, as with existing antidepressants, in the tail suspension test of mice (Non Patent Literature 4). It has also been reported that (1R,2R,3R,5R,6R)-2-amino-6-fluoro-3-[(4-fluorophenyl)methoxy]bicyclo[3.1.0]hexane-2,6-dicarboxylic acid, (1R,2R,3R,5R,6R)-2-amino-3-[(3,4-difluorophenyl)methoxy]-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid and (1R,2R,3R,5R,6R)-2-amino-6-fluoro-3-propoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid, which are (1R,2R,3R,5R,6R)-2-amino-6-fluoro-S-alkoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives, have antagonistic activity of 500 nM or less against mGlu2 receptors (Patent Literature 1 and Non Patent Literature 5).
However, the oral absorbability of the (1R,2R,3R,5R,6R)-2-amino-6-fluoro-3-alkoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives and (1S,2R,3R,5R,6S)-2-amino-3-alkoxybicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives is poor in monkeys. This suggests the possibility that the oral absorbability may also be poor in humans.
There are mainly two approaches to improvement of the membrane permeability (e.g., oral absorbability) of compounds. One is a method of changing their chemical structures themselves and the other is a method of devising a means of formulation without changing their chemical structures. The former method encompasses attaching a small modifying group such as an alkyl group or an acyl group to a reactive substituent such as a carboxy group or amino group of compounds to form them into prodrugs.
Compounds preferred as the aforementioned prodrugs are compounds that exist stably in prodrug forms before absorption, exhibit improved absorption after being formed into prodrugs and are converted to active forms chemically or enzymatically and rapidly in the small intestine, the liver and/or plasma during and/or after absorption.
However, it is difficult to develop ideal prodrugs that satisfy all of the aforementioned conditions. For example, prodrugs having an ester bond can be more likely to be hydrolyzed, which may have a great influence on chemical stability before absorption. As for prodrugs having an amide bond, a great change of the physical properties of compounds may have a great influence on membrane permeability such as oral absorbability. Further, an amide bond is less likely to be hydrolyzed, which may have a great influence on biotransformation of compounds to active forms and plasma concentrations. Furthermore, it is difficult to predict the pharmacokinetic profiles of prodrugs because enzymes that control biotransformation of prodrugs to active forms are substrate-specific and particularly, for example, the steric hindrance of a substituent inserted for formation of prodrugs prevents reaction of the enzymes. For these reasons, it is by no means easy to enhance the plasma concentrations of active forms by estimating possible improvements in the membrane permeability (e.g., oral absorbability) of prodrugs and their transformation to the active forms. There are previous reports on enhancement in the plasma concentrations of active forms by prodrugs having ester bond(s) at 6-carboxylic acid or both of 2-carboxylic acid and 6-carboxylic acid of a 2-amino-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivative which is a compound acting as an antagonist of mGlu2 and mGlu3 receptors (Patent Literatures 2, 3, 4 and 5 and Non Patent Literature 6). However, these literatures neither describe nor suggest enhancement in the plasma concentrations of active forms by prodrug compounds having an ester bond only at 2-carboxylic acid. Furthermore, the literatures neither describe nor suggest enhancement in the plasma concentrations of active forms by the prodrugs of the active forms of the present invention.